In a video display apparatus, scanning circuits are synchronized to a synchronizing component derived from the input video signal. Similarly the display device, for example a cathode ray tube has high voltage and heater supplies generated by the scanning circuitry. A video display which is capable of displaying, for example a standard definition, NTSC signal, a high definition, Advanced Television Standards Committee signal, or a computer generated SVGA signal, is capable of synchronizing to these signals which represent a range of horizontal frequencies in excess of 2:1. Although synchronized operation of deflection generated high voltage supplies is possible with the range of horizontal frequencies presented by the input selections, the complexity of design and material cost suggest that high voltage generation may be facilitated more efficiently by a high voltage generator which is separate from the deflection generation requirements of operability over the complete range of horizontal frequencies.
A high voltage power supply which is not part of a deflection generator may be synchronized with the input horizontal sync signal to reduce the visibility of power supply switching interference. However, since the input horizontal frequencies occupy nominally two frequency bands, with the higher frequency band more than double the frequency of NTSC, it is a requirement that a high voltage generator is not disturbed significantly when switching between such sync sources. Furthermore, when sync switching occurs, for example as a consequence of broadcast TV channel change or computer display resolution selection, the high voltage supply is required to remain substantially constant to avoid additional delay in the presentation of the newly selected picture source.